In recognition for the need to improve braking performance in modern day railroad freight trains, due to the advent of longer heavier cars, a continuous quick service feature was introduced in the ABDW Control Valve Device manufactured by the Westinghouse Air Brake Company. This improved version of the well-known predecessor ABD Control Valve incorporates a continuous quick service feature in an emergency portion, that is totally compatible with existing ABD Control Valves, and which includes an integrated accelerated application valve that provides the continuous quick service function.
In supplementing the quick service reduction of brake pipe pressure typically obtained in the service portion of ABD/ABDW Control Valves, the additional local reduction of brake pipe pressure obtained throughout the full service reduction period by this continuous acting accelerated application valve significantly decreases brake application times and consequently shortens train stopping distances.
The accelerated application valve portion includes a diaphragm operated control piston that is piloted by back pressure developed from the quick action chamber breather pressure. This breathing of quick action chamber pressure is the means by which the emergency piston is normally stabilized during service braking. Depending upon the rate of service reduction of brake pipe pressure, the control valve breathing pressure initiates a cycling action during which brake pipe pressure is alternately supplied to a volumetric chamber and thence is exhausted to obtain a continuous controlled local venting of brake pipe pressure. As is well-known the cycling rate and thus the degree of local quick service venting of brake pipe pressure obtained by this accelerated application valve increases as the rate of brake pipe reduction increases, it being desirable to supplement the initial, one-shot quick service obtained by the service valve to a greater degree when a heavy service application is called for than during light service applications.
The aforementioned continuous quick service feature incorporated in the ABDW Control Valve is covered in U.S. Pat. No. 3,716,276.